These proposed studies are designed to evaluate several aspects of platelet support. Included are investigations of platelet preparation; i.e. - optimize platelet yield while ensuring viability and function of platelets obtained by plateletphoresis and develop simple techniques to determine viability and sterility of room temperature stored platelets. In addition, an evaluation of indications for platelet transfusions (prevention of Cr51 labeled stool blood loss) and their effectiveness (appropriate reduction of the bleeding time for the number of circulating platelets) are planned. Platelet transfusion refractoriness is usually a manifestation of infection (viral or bacterial) or alloimmunization. Animal models will be used to determine whether available antithrombotic agents can prevent consumption associated with infection. Histocompatibility typing and platelet typing techniques (C14 labeled adenine release, Platelet Factor 3 release, radioimmunoassay and platelet-lymphocyte reactivity) will be used to screen for compatible donor-recipient combinations for platelet transfusions. As an alternative approach to donor selection, the effects of recipient immunosuppression on preventing, delaying or reversing alloimmunization will be studied. For the refractory recipient methods which will allow subsequent bone-marrow-grafting from related or non- related donors as an ultimate solution to platelet support will be investigated.